Rigid Roads Can Reduce Fuel Consumption

A new study predicts that repaving US roadways with firmer surfaces could save up to 273 million barrels of crude oil each year.

Researchers at MIT used mathematical models to examine the forces at work when a rubber tire travels over a road surface and found that cars and trucks must use slightly more energy on less-stiff streets. Just as beach sand underfoot tends to sink, the weight of a vehicle tends to tamp down softer pavement – a phenomenon known as deflection.

The end result is that tires rolling on malleable road surfaces are constantly climbing up a very small incline, which wastes fuel. Together with rough road surfaces, pavement deflection costs American drivers in total about $15.6 billion in added fuel costs and is responsible for 46.5 million metric tons of CO2 emissions.

An exaggerated example of pavement deflection. When a car travels over soft pavement, deflection happens behind the tire. That makes driving a constant uphill climb.

Study authors Dr. Franz-Josef Ulm and PhD student Mehdi Akbarian relied on data from the Federal Highway Administration that detailed the compositions of 5,643 sections of road. It’s the first time that mathematical models have been used to study the effects of road surfaces on fuel economy. Previous work relied on individual pavement samples, which didn’t offer the kind of useful data needed for a sound analysis.

“Where do you find identical roadways on the same soils under the same conditions? You can’t,” said Ulm. “You get side effects. The empirical approach doesn’t work. So we used statistical analysis to avoid those side effects.”

Though the study was funded by the Portland Cement Association and the Ready Mixed Concrete Research & Education Foundation, the authors’ recommendations were varied — though they did say a concrete layer or asphalt-concrete composite would reduce deflection. Material isn’t the only factor that determines the pliability of pavement, though, and the authors said thicker asphalt layers and sublayers would also help.

Though using better materials may initially increase the price of paving a road, Akbarian said that the reduction in fuel consumption would tilt the cost-benefit analysis in favor of building better streets and highways.

“Better pavement design over a lifetime would save much more money in fuel costs than the initial cost of improvements,” he said. “And the state departments of transportation would save money while reducing their environmental footprint over time, because the roads won’t deteriorate as quickly.”

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